Speculative Evolution Wiki:Competitions/Comp. 1: What is to be discovered/Hemicaudata
The lissamphibians have by far the patchiest fossil record of all tetrapod groups. At many stages in their evolution, there is a high level of confusion; constant debates including how old the clade really is, whether or not they form a monophyletic group, and whether the Allocaudata are closely related to salamanders. Making a few assumptions, we can tell about how old each major clade is. A figure of 250 million years seems appropriate for Salientia, with the crown group Anura being only slightly younger; salamanders are their probable sister group, so Caudata is also about 250 million years old. Except for one thing – salamanders are only 164 million years old according to the fossil record. Salamanders are not the only case of this extreme difference between the appearance in the fossil record and the approximate age going by phylogenetics. The Allocaudata, a relative recently extinct lissamphibian order, is thought to be the sister taxon to a clade consisting of Caudata and Salientia. That would make them older than 250 million years old, likely being survivors of the Permian-Triassic extinction. The case with Allocaudata seems even more extreme than that of salamanders considering their appearance in the fossil record was about the same time as that of salamanders. The appearance of the earliest caecilian in the fossil record is in the Early Jurassic, but the theoretical appearance of caecilians is hard to date. Although one hypothesis states that caecilians are unrelated to the Batrachia, there is a good chance they are the sister group of the Batrachia. If that is the case, stem-caecilians should have evolved in the mid Permian. The most plausible explanation for the lack of lissamphibians in the fossil record is simply that they weren't common. But suppose one group took it a step further, and never got diverse, completely escaped the fossil record, and is hiding among the modern fauna. This group is the Hemicaudata. Description The Hemicaudata are members of the Salientia, and thus closely related to true frogs. They resemble frogs at least externally, but internally they retain many characteristics not found in any living frog. Some of the more primitive characteristics found in Micrura are sixteen vertebrae, far more than the nine or ten in modern frogs; a short tail retained through adulthood, containing five of these vertebrae; and the inability to jump, or at least in Micrura asalientia. Because of the "excessive" vertebrae, two more than in Triadobatrachus, the Hemicaudata have a longer and thinner body than modern frogs. The tail of hemicaudates is no more than a stub, unlike their more distant salamander relatives, but adult frogs have no tail at all. The inability to jump, or partial inability in the case of Micrura microscopica, is due to the relatively weak muscles in their back legs. The heads of Micrura are also thin, almost as thin as those of salamanders, although still clearly frog heads. Their skin is brownish-green in colour, and rough like that of a toad. This skin appears not to be ancestral to frogs, but rather evolved over the apparently long duration of hemicaudate existence because of the habitats which supported the elusive creatures. The nostrils lie on the very tip of the flattened, thin head. The eyes lie on top, not on the sides of the head; a strange and apparently disadvantageous position, but the only one available to these creatures, not finding a way to evolve a less flattened head. Micrura asalientia has more stocky legs, giving it the inability to jump whatsoever. However, Micrura microscopica has skinny, powerful legs, allowing it to at least jump while running in its own strange manner, with the two front legs lifting at the same time, than lifting the back legs just before the front ones hit the ground. This motion is actually quite quick, but the distances and heights achieved in one jump are far less than those of frogs. M. microscopica measures 1 to 1.5 centimeters in length; while M. asalientia is much longer, at 5 to 6 centimeters. Discovery While hiking through the gigantic Sơn Đoòng caves in Vietnam, a team of researchers spotted a fast moving frog-like creature. This was their second trip to the caves, and although the first time they had reached the "end" of the caves, they had only just begun to explore it. At first, this creature appeared to just be a normal frog, but through common knowledge, no frog known hopped like this. And so the team attempted to catch the creature, and were successful after spotting a couple more individuals. They were geologists, not herpetologists, however, so they simply captured it and brought it to a team of zoologists to study. The "frog" appeared to be more basal than any other living salientian, and the tiny creature, measuring just over a centimeter long was given the binomial Micrura microscopica, because of its small tail and small size. This proved to be a major discovery, and the team of geologists was sent back to search for any other short-tailed frogs, so called Hemicaudata. The team figured since these were amphibians they would surely congregate along the river that was known to flow through the Sơn Đoòng caves. However, after an entire day of searching along almost the entire length of the river, no more of the short-tailed frogs were found and only two individuals of Micrura microscopica were found, these ones away from the river. It seemed they were more terrestrial, like toads, and they would only go to the river during mating season. So, when the geologists returned unsuccessfully, the zoologists suggested going during the mating season. In the tropical climate, the zoologists assumed the frogs would begin to breed around July. So, the following July, the geologists went on their fourth excursion through the Sơn Đoòng caves, and returned with a much larger short-tailed frog, the only one of that species they could find, but a new species nonetheless. This was given the binomial Micrura asalientia, because of its inability to hop. The next research these zoologists were to do was how and when these strange frog relatives evolved. Evolution The first of the mysteries to solve about the strange short-tailed frogs was whether they were actually basal members of Salientia, or if they were somewhat degenerate frogs. Through genetic dating, however, it was confirmed that the last common ancestor of either of the short-tailed frogs and ordinary frogs was over 210 million years ago. Therefore, Hemicaudata went from a nomen nudum to an official order containing only two species. No other Hemicaudata were known from the fossil record, although suggestions that each of Triadobatrachus, Czatkobatrachus, and Prosalirus were part of Hemicaudata were put out. However, all three were rejected, the former two because they are considerably older than the LCA of Anura and Hemicaudata by genetic dating, and the latter one because of its small numbers of vertebrae and its ability to hop. The hopping ability of Prosalirus brings up another question, however, and that is whether the hopping ability of Micrura microscopica evolved secondarily and it was closely related to Micrura asalientia, or if the hemicaudates were a polyphyletic grouping of converging basal salientians. Both morphological evidence, compelling enough but better supplemented by genetic evidence, show that the hopping ability of M. microscopica evolved secondarily, and even more is unrelated to the hopping mechanisms of Anura. The two species are shown to be separated by 35 million years of evolution, and hence have been evolving in the same habitat since the Oligocene, otherwise the two would be separated not only by tens of millions of years of evolution, but also by great distances. There appears to be some sort of symbiotic relation between the two, although no evidence has been documented thus far. The final question to be solved is "What is Hemicaudata defined as?". Some define it simply as the LCA of the two Micrura species, but the best definition is everything more closely related to Micrura microscopica (the type species) than to Anura. Still, for now no other hemicaudates are known, and Hemicaudata is synonymous with Micrura and Micruridae, but that doesn't mean they have to be. If a fossil hemicaudate is found, than the order may no longer be synonymous with the genus and family. However, even if no prehistoric documents of the Hemicaudata are known, historic ones may be. The Dong Son people of Vietnam believe in a water god represented by a frog, and in honor of that have many depictions of frogs on their drums. Although this evidence is ambiguous, it could mean two things which add a level of complication to the hemicaudate problem – that the two extant species were known by the Dong Son and were nearly hunted to extinction, or that these tailed frogs made up a large part of the amphibian fauna of Vietnam in the past, before being reduced to two species in historic times. In fact, some of these extinct species may have been almost fully aquatic, hence the association with water. Still, many other frogs are known in Vietnam, for example, two other species in the genus Theloderma were discovered only in 2011. This large diversity of frogs makes it unlikely that the hemicaudates were common in the past, even if they were known to the Vietnamese. Also, the competition from other frogs makes it unlikely that their range was ever wide, combined with the fact that obviously none are known from the fossil record. Status The two hemicaudate species appear to be endangered, M. asalientia more so than M. microscopica, however they have not been evaluated by the IUCN. Considering the very recent discovery of the Sơn Đoòng caves, it is unlikely that this is caused by humans, unless the Dong Son did indeed endanger the two species. Still, many examples show that endangered species don't always go extinct, and perhaps throughout evolutionary history, the Hemicaudata have always been on the brink of extinction, outcompeted by their contemporaries, the frogs. Or maybe the fossil record is so badly preserved that at certain points in their history, the hemicaudates were not rare, possibly even outcompeting frogs in certain isolated areas. Neither of these theories can be proven or disproven until we learn more about these unusual amphibians, though, and so we retain much ambiguity about this group. Ecology and Life Cycle Little is known about the ecology and life cycle of these mysterious amphibians, but scientists have pieced together several intriguing solutions to problems involving what is not known about these creatures, coming up with assumptions related to their life cycles. However, before the radical theories are explained, the straightforward theories will be. The general approach is that these creatures live much like frogs, breeding and laying eggs in the water, in this case the river running through the caves. Their diet consists of insects, specifically beetles and flies (although this has not been challenged, this was a laboratory observance, not an observance of the creatures in their natural habitat). Micrura microscopica hunts insects with speed, while the more camouflaged Micrura asalientia lacks speed, and hunts with stealth. Both species appear to have great hiding strategies, and quite plausibly they used these not yet documented strategies to escape being discovered in large numbers, probably assuming humans as predators. This hiding strategy, in yet another assumption (although still a rather straightforward one) is used to escape usual predators as well. But this provides no explanation as to how and why these species have stayed in the same habitat since the dawn of the Oligocene. This next theory assumes the dietary and predation-related aspects of the previous theory, but provides a very radical explanation as to the species staying in the same habitat since the Oligocene, in the process explaining ambiguities about the life cycle of Hemicaudata. This theory is based on two facts about the discovery of the genus: #No tadpoles or larvae of any sort were found; and #Only Micrura asalientia was found near water. The theory can be summarised as follows. Micrura microscopica is no more than a juvenile form of Micrura asalientia (still, Micrura microscopica would be a senior synonym of M. asalientia, but this is not relevant to the theory). The hopping ability of M. microscopica is therefore related to size, and nothing else, being lost in the larger adults. Yet of course no amphibians grow after losing their gills and metamorphosising. So, according to this theory, these amphibians lack a larval stage, and M. microscopica (here used to refer to what in this theory are simply a life stage) is what hatches out of the eggs. The evolution of this over at least 210 million years is not implausible, but the most radical part of the theory explains why the genetic dating was wrong. Apparently, several "races" of this species have evolved, and even through 35 million years, an individual of this species has remained able to breed with any other individual of the species. So, taking that it's unlikely the individual of each "species" (again, simply a life stage according to this theory) which had its genetic code examined was as far apart as possible genetically from the other, the unified species would be older than 35 million years, and yet had not yet diverged. Or, a variant of this theory even states that it had indeed diverged, and there may still be multiple species – and M. asalientia may remain valid – in which case the type specimen of M. microscopica would be designated as part of the M. microscopica species (with some of the adults also being part of this species), and if the type of M. asalientia cannot breed with the type of M. microscopica, then this will remain a species (including some probably thought to be M. microscopica). Overall, the first variation of the theory probably makes too many assumptions about evolution in action to be considered plausible (at least until we learn more about evolutionary processes), while the second (probably more plausible) could simply make this situation more confusing. In fact, the second one could be proven, especially if more than two species incapable of interbreeding with each other are found. And although the second variation of the theory gives less of a reason as to why they stayed in the same habitat, a simple explanation of "they stayed the same in niche, while separating genetically" is acceptable. And so, it seems that the two main contenders for this theory are the mainstream one and the second variation of the radical theory. But it's only a matter of time until one is proven: their life cycle shouldn't be any longer than those of usual frogs, and so if one of the apparent juveniles is kept long enough, it will either become the mainstream M. asalientia, proving the radical view, or simply stay as M. microscopica proving the mainstream view. Still, this remains undecided. And so, I have explained in detail to you one of the strangest recent zoologic discoveries, basal frog relatives with sixteen vertebrae, many other primitive features, and many ambiguities about how they live and how they evolved. And yet, one fact remains to be explained – this is no more than a work of fiction. Hemicaudata